by S. Aull, sarah.aull@cern.ch; E. Jensen, erk.jensen@cern.ch; A. Macpherson, alick.macpherson@cern.ch; G. Rosaz, rosazguillaume.rosaz@cern.ch;
A. Sublet, alban.sublet@cern.ch; W. Venturini Delsolaro, walter.venturini@cern.ch

CERN has a long history in superconducting radio frequency (SRF) technology. What started with a few SRF cavities in the 1980s has now become a major field of activity. Today CERN is not only running a variety of superconducting RF installations and projects but is also preparing for a new collider, to follow the Large Hadron Collider (LHC), that will require pushing the limits of current technologies for better performance and lower cryogenic consumption.

LEP2 (1989-2000), with almost 300 superconducting cavities, remains the biggest SRF system in the world and was the first machine that made use of niobium coated copper cavities instead of cavities made from niobium sheet. Such thin niobium films, of about 1.5μm thickness, are deposited by magnetron sputtering and have several advantages such as thermal stability and significant savings in raw material.

The high thermal conductivity of the copper substrate allows operation at 4.5K instead of 1.8-2K as customary for bulk niobium. However, losses in the cavity wall increase much more with the RF field for niobium films than for bulk niobium. This has restricted the application of niobium on copper (Nb/Cu) technology to machines requiring only low accelerating gradients such as the LHC at CERN or the Acceleratore Lineare Per Ioni (ALPI) at INFN Legnaro.

In 2006, Nb/Cu technology was chosen for the High Intensity and Energy upgrade of CERN’s ISOLDE facility (HIE-ISOLDE). Its superconducting linear accelerator increases the energy of radioactive ion beams up to 4.5A/q from 3MeV/u up to 5.5MeV/u with the use of two cryomodules in a first phase, and up to 10MeV/u after a later addition of two more cryomodules.

Each cryomodule houses five superconducting quarter wave resonators and one superconducting solenoid. The cavities are fabricated from OFE (Oxygen-Free Electronic) copper and coated with niobium by means of a DC bias sputtering method. The cavities work at a resonance frequency of 101.28 MHz at 6MV/m and are operated at 4.5K. The cryomodule is of the common vacuum type: the beam vacuum is the same as the insulation vacuum, creating a challenge during the assembly process of avoiding dust contamination. Cavities are conduction cooled by a static boiling helium bath and surrounded by a thermal shield that is actively cooled with helium gas at 70K. Researchers installed the first cryomodule in April 2015 and thereafter ran thorough qualification tests. The commissioning campaign culminated in October 2015 when the first radioactive beams were successfully accelerated and delivered to the users.

The development of superconducting cavities for HIE-ISOLDE re-launched SRF activities at CERN, in particular the development of niobium coatings. Today, CERN runs a variety of cryogenic RF test locations to meet the demands of different SRF projects and activities.
Experimental hall SM18, for example, provides four large vertical cryostats suitable for the RF testing of bare cavities and two horizontal bunkers for validation of fully assembled RF cryomodules. This infrastructure is used to test LHC cavities, HIE-ISOLDE cavities, crab cavities for the High Luminosity Upgrade of LHC (HL-LHC) and 704 MHz, five-cell cavities of the high gradient program.

For the validation of RF cavities made from high purity niobium, testing is typically done in the 4m deep vertical cryostats in a superfluid helium bath at between 1.8 and 2K. Rapid controlled cooldown from room temperature is needed to avoid generation of detrimental niobium hydrides in the RF surface layer, and with a pre-cooled thermal screen, cooldown rates of 15K per minute are achieved. In addition, to ensure good RF performance, care is taken to avoid trapped magnetic flux when cooling the niobium cavity through the superconducting transition at 9.2K. A fixed spatial temperature gradient over the cavity volume is maintained as it is cooled through the transition by balancing cryostat heater power against cooling power from filling with 4.5K helium, ensuring any remnant ambient magnetic field is fully expelled from the superconductor.

Once superconducting, researchers can condition the RF surface of the cavity and evaluate its performance. Key performance indicators include the susceptibility of the cavity to quench and the onset of field emission phenomena—whereby radiation (mostly X-rays, but extending to the gamma region for multicell structures) is generated by electrons extracted from the RF surface and accelerated in the cavity’s electromagnetic field. To quantify the severity of u defects, scientists localize quench spots by triangulation using an Oscillating Superleak Transducer (OST) array, sensitive to the second sound wave from the quench. Once a quench spot is localized, the RF surface is inspected for defects in the post-test phase, and where possible, the defect is repaired.

CERN’s central cryogenic laboratory provides the infrastructure and technical support necessary for niobium film R&D activities and RF testing. A 150 liter cryostat is used to test R&D objects like 1.3 GHz cavities and CERN’s Quadrupole Resonator, a sample test cavity that allows RF characterization of flat samples at typical SRF cavity frequencies.

Elliptical cavities are the most common cavity geometry with 1.3 GHz as a common frequency. Therefore, and because of its compact size, a 1.3 GHz single cell cavity is the ideal development object. Such cavities are not only used to improve the standard magnetron sputtering of niobium on copper but also to further the development of niobium coating technology. Energetic condensation techniques like High Power Impulse Magnetron Sputtering (HiPIMS) grow thin films of higher quality and are expected to result in improved SRF performance. Pushing the limits of niobium thin films is a key challenge for the electron-positron accelerator, one of the scenarios explored by the study for a Future Circular Collider (FCC).

The FCC is a design study for a post-LHC particle accelerator at the high energy frontier with a conceptual design report due to be delivered by end of 2018. Using niobium coated cavities for FCC would allow an operation temperature of 4.5K without losing thermal stability. Moreover, the requirement for relatively low RF frequency (400-800 MHz) will result in big accelerating structures with a cell diameter between 37.5cm (800 MHz) and 75cm (400 MHz). Building hundreds of structures of that size out of bulk niobium would result in excessive material cost. The niobium film technology comes with the benefit of saving a large amount of high purity raw niobium material. Nonetheless, the field limitation of the current Nb coatings make these cavities inefficient at fields required for FCC.

Recent results on an energetically condensed Nb/Cu film, produced and tested in collaboration between Jefferson Lab and CERN, revealed a significantly better performance compared to the standard magnetron sputtering. Several laboratories worldwide have also moved beyond niobium in general, launching coating activities that use alternative materials for SRF applications. A15 materials such as Nb3Sn and V3Si, for example, have the potential to lower even more cryogenic losses in a cavity. Tests with Nb3Sn fabricated by reactive evaporation, as done at Cornell, indicate that it could already outperform bulk niobium, though this largely depends on a high quality bulk niobium cavity as a substrate. Herein, researchers are seeing great potential from coating Nb3Sn onto a copper cavity and subsequently profiting both from thermal stability and reduced material costs.

Labs are also investigating new fabrication processes for seamless cavities. Researchers view electro-hydroforming, where the metal sheet is deformed by a shock-wave, as an especially promising candidate as first results have shown excellent mechanical properties. Also within the FCC framework, a collaboration between INFN Legnaro, STFC Daresbury and CERN is dedicated to the development of a process to fabricate 800 MHz seamless cavities by spinning. The team coats these cavities with a high quality film to test and thereafter further understand the relation between coating parameters and SRF performance.